I did look it up and all, but I'm finding "I wouldn't risk it!" VS "I use it". Does anyone know if I could use it?

YES I know I could make the craft mesh one and no I don't want to. I don't have it, don't feel like getting it, and have a ton of coroplast lying around

I would like to divide my 20 long, for three males. It's less stress on them if they do not see their neighbor - especially the one in the middle! Then I'm dividing my 10 gallon with the divider I already have.

And can you say WHY it can/cannot be used? The one I have is plain, nothing printed on it, been used for nothing, it's just lying around.

From what I'm reading, coroplast is made of a plastic that is considered safe (in this usage: no known health hazards). In fact, it appears that styrofoam is potentially more of a risk than coroplast, and I know people use that for carved backgrounds and such (albeit I believe they seal the foam).

Just about any plastic container you buy will have a small triangle with a number or symbol in it. This is a guide to what that symbol means.. which is helpful in deciding what is safe for fishies.

Coroplast is made up of polypropylene, Which has Resin identification code 5, meaning it can be used for Auto parts, industrial fibers, food containers, and dishware. I tend to believe that food containers, if washed properly, are safe for fish.

I went by the identification codes as well, though I used this page instead.

Styrofoam/sytrene leaches when exposed to hot liquids, fatty oils, and alcohol (likely not an issue in an aquarium, but also not something that you have to worry about with coroplast). Actually, PVC and vinyl are also "less safe" than PP and people use pieces of PVC pipe in their aquariums all the time.

I don't know as I would go so far as to say sytrofoam and PVC and vinyl and stuff are dangerous, but they're not as safe as some other plastics, basically.

And if that's the best reason they have against using it, I'd disregard their concerns. Besides, one way or another the fish come in plastic, don't they? And acrylic is a type of plastic as well.

"Code 3: Polyvinyl Chloride, or PVC / Vinyl. This material, used often in flooring and shower curtains, as well as household water pipes (primarily for evacuation only - they should never be used to bring fresh water into the home), used to be used in cling wraps. The plastics industry is adamant that the type of PVC currently used in cling wraps does not contain the phthalates that are known endocrine disrupters. However, these phthalates may still be present in PVC bottles and toys. There was recent information that many baby teethers were also made from PVC, due to its soft flexibility. PVC or vinyl items should never be given to a baby or child who may put them in his or her mouth."

The emphasis is my own, but it states that PVC contains phthalates which "are known endocrine disrupters." Whether it affects fish and aquatic animals the same it may affect humans, however, I can't say with 100% certainty as I haven't looked into it.

As regards the coroplast, you can clean it with hot water if you feel it's necessary: as said, the plastic made to use it (I looked it up, it's code 5) has no known health hazards, including when exposed to hot water. I suspect it wouldn't even leach dye.